This invention relates to moisture sensors for detecting moisture such as rainfall and dew as well as other matter such as dirt, dust, salt film and the like on windows. The invention is especially adapted for vehicle windows to control accessories such as windshield wipers and the like. More particularly, the invention concerns a control for a moisture sensing apparatus mounted on one side of a window panel for detecting moisture and other matter on the opposite side of the window panel while maximizing the area sensed and minimizing the overall assembly size.
Various types of moisture sensing apparatus have been used on vehicles to control accessories such as windshield wipers. Optical moisture sensors have been found particularly useful since they require no special coatings or other structure on the exterior of the window or vehicle for physically contacting rainfall or moisture. One such device employs an infrared emitter and detector which senses infrared energy emitted from inside the windshield or window and refracted and reflected back at decreased levels when moisture or other matter is present on the outside of the windshield to a detector also mounted on the interior of the vehicle. Such a sensor is disclosed in U.S. Pat. No. 4,798,956 and includes an emitter which emits pulses of infrared energy toward the inside surface of a window at an incident angle. The radiant energy is refracted into the window at the inside air/window surface and reflected off the air/outside window surface back through the window where it is again refracted and detected by a detector/sensor positioned along a reflection angle equal to the incident angle. The emitter and detector are spaced and positioned at predetermined locations and angles all as set forth in U.S. Pat. No. 4,798,956.
An improved control for such moisture sensing device is disclosed in copending, commonly-assigned U.S. patent application Ser. No. 07/317,288, filed Feb. 28, 1989 by Kenneth L. Schierbeek, Mark L. Larson and Kenneth Schofield, entitled "CONTINUOUSLY ADAPTIVE MOISTURE SENSOR SYSTEM FOR WIPER CONTROL." That control is used with a moisture sensing device positioned on a portion of the windshield cleared by the wiper and provides an adaptive moisture sensor system. The disclosed control includes moisture sensing means responsive to the presence of moisture for producing a moisture signal, reference signal generating means for generating a reference signal and comparison means for producing an output to the vehicle wiper system that indicates the relationship of the moisture signal to the reference signal. In this control, the reference generating means is responsive to the moisture signal for continuously adapting the reference signal toward the moisture signal to cause the control to adapt to changes in the system as a result of component aging, mounting tolerances and the like. During rain conditions, the reference generating means adapts the reference signal to the moisture signal at a faster rate in response to decreasing sensed moisture than in response to increasing sensed moisture such that the control adapts to the driest sensed condition of the windshield. The reference generating means additionally adapts the reference signal at a fast rate, to offset the reference signal and terminate dry wiping, when the moisture signal becomes stable due to a soiled windshield. The fast adapting functions are disabled during non-moisture conditions to reduce the tendency of such a control to adapt to rapidly changing environmental factors such as overhead street lights.
Use of the optical moisture sensing devices revealed that the area of the window being sensed was relatively small, and that the probability of moisture impinging on the sensed area was likewise small, especially in light rain conditions. Thus, in conditions where a relatively small and scattered number of raindrops strike the vehicle and its window areas, the probability of such a drop falling on the sensed area and being indicated to control the desired vehicle accessory (such as the windshield wipers) often prevented operation of the wipers until long after significant rain had begun to fall.
In the above referred to Schierbeek et al. application, a plurality of moisture sensing units are provided, each of which includes a moisture sensor, a reference generating circuit and a comparison means. These units are positioned at selected locations on the windshield within the area cleared by the wiper(s). A master control receives output signals from all of the moisture sensing units and responds to any sensing unit output indicating the presence of moisture by enabling the fast adapting function for each and every moisture sensing unit.
In order to obtain a reliable indication of a moisture condition, however, the desired number of such moisture sensing units occupies a large portion of the windshield. Therefore, a need was recognized for an improved moisture sensing device of the optical type which would maximize the window area being sensed while minimizing the size of the sensing assembly.
This need is addressed in copending, commonly-assigned U.S. patent application Ser. No. 377,942 filed on even date herewith by Desmond J. O'Farrell and Kenneth Schierbeek, entitled "VEHICLE MOISTURE SENSOR AND MOUNTING APPARATUS THEREFOR." This application discloses a moisture sensor providing an increased sensing area while minimizing the assembly size through the inclusion of two or more energy emitting diode arrays (emitters) and energy detecting photosensor arrays (detectors) arranged in a stacked relationship. Radiant energy barriers reduce the reflection of radiant energy from the first emitter and the inner surface of the windshield to the first and second detector and from the second emitter and the inner surface of the windshield to the first detector.